Issue
Eur. Phys. J. Appl. Phys.
Volume 85, Number 2, February2019
Materials for Energy harvesting, conversion and storage (Icome 2017)
Article Number 20401
Number of page(s) 11
Section Nanomaterials and Nanotechnologies
DOI https://doi.org/10.1051/epjap/2019180312
Published online 28 February 2019

© EDP Sciences, 2019

1 Introduction

Nowadays, zinc oxide is becoming an outstanding candidate for nanoscience and nanotechnology. It has exclusive advantages over other competitors (tin oxide (SnO2) titanium oxide (TiO2)), such as abundance in earth crust, non-toxicity, low-cost material, chemical stability, high lattice parameters, a = 0.325 nm and c = 0.521 nm, and good transparency in the visible and near-infrared spectral region. In addition, zinc oxide conferred a large excitation binding energy (60 meV), that is 2.4 times the binding energy of GaN (25 meV), high electron mobility [1,2] and a direct wide band gap semiconductor (E g = 3.37 eV at 300 K, n type) [35].

Owing to these exclusive properties, zinc oxide has gained intensive attention for myriad applications such as optics, mechanics and biomedical sensing nanodevices, gas and chemical sensors, transducers, transparent electrodes in solar cells, photocatalysis, UV photoconductive detectors, UV lasing and short-wavelength optoelectronic devices [614]. The physical properties of ZnO thin films were governed by deposition methods, synthesis parameters, annealing treatments and doping. Among these factors, doping ZnO with selective elements can offer an effective method to improve their structural, morphological, optical, electrical, magnetic and PEC properties without any change in the crystalline structure. ZnO can be doped with a wide variety of elements to meet the demands of different applications such as elements from group-IA elements (Li, Na and K) [1517], Group IIA (Mg) [18], Group IIIA (Al, Ga, In) [1921]), Group VA elements (N, P) [22,23], Group VII (Cl) [24] and transition metals (Mn [25], Fe [26], Co [27], Ni [28], Cu [29,30], Ag [31]) and so forth.

Among dopants, silver is one of the most promising metals to enhance structural, morphological, optical and electrical properties. Several research groups have investigated the interface of doped ZnO/electrolyte [3235] and demonstrated the importance of these semiconductors as photoactive materials in thin films-based PEC solar cells which have wide applications because of their low fabrication cost, high-throughput processing techniques and ease of junction formation with an electrolyte.

To date, undoped and silver-doped ZnO (SZO) nanostructured thin films have been prepared by various techniques such as pulsed laser deposition (PLD) [36], RF magnetron sputtering [37], e-beam evaporation techniques [38], atomic layer deposition (ALD) [39], hydrothermal method [40], electrochemical deposition [41], spray pyrolysis technique [42] and sol–gel process [4345]. Especially, the sol–gel method has attracted great attention due to its unique advantages, including low cost, simple deposition equipment, easy adjusting composition and dopants and lower crystallization temperature.

The effects of silver doping on structural, optical and electrical properties of SZO thin films have been extensively studied in literature. However, there is a scarcity of reports dealing with the effect of Ag doping on PEC properties of SZO thin films by using simple and economical sol–gel method. Also, the correlation between PEC and photoluminescence (PL) measurements of Ag-doped ZnO/ITO photoanode has not been reported earlier.

In this work, undoped and silver-doped ZnO thin films have been deposited onto indium-doped tin oxide (ITO) conductive substrates by sol–gel method via spin coating. Then, a wide investigation of Ag incorporation influence on structural, morphological, optical and PEC properties of SZO thin films have been developed. Thereafter, a detailed study has been devoted to the correlation between photoelectrochemical (PEC) and PL measurements of Ag-doped ZnO/ITO photoanode.

2 Experimental details

2.1 Sample preparation

The fabrication process of silver-doped zinc oxide films is the same as that reported in our previous paper [46]. Undoped and silver-doped zinc oxide thin films were deposited onto indium-doped tin oxide (ITO) substrates by sol–gel technique via spin coating process. All substrates were previously cleaned in an ultrasonic bath with a succession of solvents, acetone, ethanol and deionized water, for 10 min each in order to eliminate any dust or contaminants.

The chemical system used involves a precursor for Zn, zinc acetate dihydrate (Zn(CH3COO)2-2H2O, 99.9%, Merck) and for Ag: silver nitrate (Ag(NO3). Precursors were dissolved in a solution of absolute ethanol as solvent, and monoethanolamine (NH2CH2CH2-OH, Sigma Aldrich) as a stabilizer. The molar ratio in the solution was varied to give a ratio of [Ag/Zn] equal to 1, 2 and 5%. The resultant solutions were stirred at 60 °C for 2 h and cooled at room temperature for 24 h to yield a clear, transparent and homogeneous ZnO aqueous solution. The solutions were, then, employed in the zinc oxide thin films synthesis using a spin coating technique. All films were spin-coated on ITO substrates. Spinning speed was kept at 3000 rpm while spinning time was 30 s. Deposited film was dried at 300 °C for 10 min. After drying, more coats can be deposited by the same simple process. The procedure from spin coating to preheating was repeated for several times in order to obtain the desired film thickness. In this case all samples had a total of four coats. After deposited, the films underwent a thermal treatment in a tubular furnace. Films were treated for 2 h at 550 °C in air atmosphere in a MTI corporation-type chemical vapour deposition furnace (CVD).

2.2 Characterization techniques

In order to study structural and morphological properties of silver-doped ZnO (SZO) films, XRD measurements were done using the X-ray diffractometer (Phillips (PW3719) X'pert materials research) with a CuKα radiation (λ = 1.5418 Å), and SEM images were analysed using scanning electron microscopy (SEM) by means of a Raith PIONEER system.

The thickness of the elaborated SZO films were carried out using a Dektak XT Bruker profilometer.

The optical studies in the UV-visible range were recorded using a LAMBDA 950 UV/vis/NIR spectrometer from Perkin Elmer within 250–800 nm wavelength range.

PL measurements were carried out using a laser source line with wavelength of 266 nm for the excitation of Ag-doped ZnO samples. The light emitted by the sample was detected through a 250 mm Jobin Yvon monochromator by a GaAs photomultiplier associated with a standard lock-in technique.

The PEC measurements were performed using a computer-controlled Autolab potentiostat metrohm AUTOLAB (30 PGSTAT) in a conventional three-electrode arrangement with the deposited ZnO or Ag-doped ZnO thin film as a working electrode, a platinum as a counter-electrode, an (Ag/AgCl) as a reference electrode and a 0.5 M Na2SO4 aqueous solution as an electrolyte. Figure 1 shows the PEC experimental equipment. The PEC characteristics were measured under constant illumination of 200 mW/cm2 from a Xenon lamp (CHF-XM-500 W). The effective area of the photoanode using for illumination was 1 cm2.

Electrochemical impedance spectroscopy (EIS) measurements were carried out at the open-circuit potential in dark and under illumination. A sinusoidal AC perturbation of 10 mV was applied to the electrodes over the frequency range of 0.01–105 Hz.

thumbnail Fig. 1

Schematic diagram showing photoelectrochemical current measurement.

3 Results and discussion

3.1 Structural properties

The X-ray diffraction (XRD) patterns of pure and silver-doped ZnO thin films are shown in Figure 2. All the observed positions peaks, at around 31°, 34°, 36°, 47°, 56° and 62°, have been identified using the standard JCPDS 036-1451 ZnO card indicating that all SZO samples have polycrystalline hexagonal würtzite structure. The corresponding Miller planes are (100), (002), (101), (102), (110) and (103), respectively. There are no other crystallized phases, neither of silver nor of its oxides can be observed in any of the samples.

It can be seen from Figure 2 that the (002) peak intensity is gradually improved with the increase of Ag doping content from 1% Ag to 5% Ag doping content. This means that all SZO exhibit preferential orientation growth along the c-axis perpendicular to the substrate surface. Also, this trend demonstrates that the crystallinity of samples was enhanced with further increase in Ag concentration. The similar trend of enhanced crystallinity was reported by Li et al. [47] as yielded by the increase of (002) peak intensity in the XRD patterns of sol–gel deposited Li: ZnO thin films with increasing the Ag co-doping contents. Several research groups have reported the shift of (002) peak position to lower angle [4851] indicating that most of Ag dopants substitute Zn2+ ions. In our case, an obvious shift toward higher angle of (002) peak position was noted with increasing Ag doping level in SZO thin film. The similar trend was also observed by Karki et al. [52] and Tripathi et al. [53] confirming that more of Ag ions are either settling in interstitial sites or making clusters.

To reach some valuable information regarding the enhancement of ZnO thin films structure, some calculations have been done. Indeed, the particle sizes D values are estimated from (002) diffraction lines of all ZnO:Ag thin films using Debye–Scherer formula as follows (Eq. (1)) [54]:(1)where k = 0.90 is the Scherrer constant [32,42,46,5557], β 1/2 is the width at half-maximum and λ = 1.5418 Å is the wavelength of CuKα radiation. As seen in Table 1, the grain size is found to increase from 21.99 to 32.60 nm with the increase in silver concentration.

The strain (ε), which is an interesting structural parameter of Ag-doped ZnO thin films, was calculated using equation (2) [58]:(2)where β 1/2 is the full-width at half-maximum of (002) peak and θ is the Bragg angle. It can be seen from the calculated values of the strain mentioned in Table 1 and from Figure 3, showing the dependence of both grain size D and the strain ε to Ag amount. Of silver-doped ZnO thin films, that contrary to grain size, the strain decreases from 53.29 × 10−4 to 35.74 × 10−4 with the increase of silver concentration.

The dislocation density (δ), defined as the amount of defects in the film, was estimated using equation (3) [59]:(3)

In contrast to the crystallite size, the dislocation density values, which are listed in Table 1, have a tendency to decrease from 2.06 × 1015 (lines/m2) for pure ZnO to 0.94 × 1015 (lines/m2) when the Ag content was increased up to 5% Ag.

thumbnail Fig. 2

XRD spectra of Ag-doped ZnO thin films deposited on ITO substrates at different Ag contents (Ag = 0, 1, 2 and 5%).

Table 1

The microstructural parameters evaluated from XRD measurements such as position peak, lattice parameters (a, c), grain size (D), microstrain ϵ and density of dislocation δ for Ag-doped ZnO thin films.

thumbnail Fig. 3

Crystallite size and residual stress of silver-doped ZnO thin films.

3.2 Morphological properties

Scanning electron micrographs of silver-doped ZnO thin films are shown in Figure 4. The films show uniform and dense morphology overall the surface. This morphology consists of spherical aggregate with an average grain size around 48 nm for undoped ZnO. It is obvious from these micrographs that the surface morphology of pure ZnO and ZnO:Ag films was slightly modified with doping. After doping, there is an increase in grain size reaching 66.9 nm for the ZnO:Ag 2% sample and with further increasing the silver doping content to 5% the film becomes relatively more compact and uniform with tiny grain size.

So far, the published results on the effect of Ag-doping on ZnO crystal growth are inconsistent. For instance, Liu et al. [60] found that the Ag doping led to a smaller ZnO grains; Zhang et al. [61] deemed that the grain sizes in Ag-doped ZnO thin films were more non-uniform than those in pure ZnO thin films. Nevertheless, Liu et al. [62] revealed that the Ag-doping resulted in smaller ZnO grains when the Ag-doping concentration was low while Ag nanoparticles were formed when the Ag-doping concentration was high. Xu et al. [43], however, have reported that the rise of Ag-doping concentration leads to a gradually increase in ZnO grains. It should be noted that these differences mainly arises from the various lattice sites of Ag in ZnO due to different film deposition methods and annealing treatments.

thumbnail Fig. 4

SEM micrographs of the sol–gel-deposited Ag-doped ZnO thin films at various Ag contents. (a) ZnOpure (b) ZnO:Ag 2% and (c) ZnO:Ag 5%.

3.3 Thickness measurements

The thickness values of the elaborated SZO films are listed in Table 2. It can be seen that the undoped ZnO thin film has a thickness around 1128.2 nm. Ag incorporation into ZnO thin films leads to an increase of thickness from 1184.6 for 1% Ag doping content to 1506.1 nm for 5% Ag.

Table 2

Thicknesses of undoped and Ag-doped ZnO thin films.

3.4 Optical properties

The optical transmission spectra recorded in the wavelength range 250–800 nm are shown in Figure 5a. It is obvious that the films are highly transparent in the visible region (70–84%) and all the spectra exhibited weak fluctuations mainly due to interference phenomena in the samples. With rising Ag doping concentrations, the average transmittance gradually decreases. This decrement in the transmittance of the Ag-doped ZnO thin films with further increase in doping concentrations from 1% up to 5% Ag content may be due to the increase either of surface roughness or in scattering of photons by crystal defects created by Ag doping which agglomerated in cluster in the ZnO layer. The decrease in transmittance after Ag incorporation in SZO thin films was also reported by several research works [35,42,52,63].

The band gap is preferred to be evaluated from the optical transmission spectra. In order to calculate the optical band gap energy of Ag-doped ZnO films, the absorption coefficient can be estimated as follows (Eq. (4)) [57]:(4)

The optical band gap of Ag-doped ZnO thin films can be calculated by Tauc's formula (Eq. (5)) [64]:(5)where α is the absorption coefficient, A is the constant, h is the Planck's constant, ν is the photon frequency, Eg is the optical band gap and n is the 1/2 for direct band gap semiconductors. Figure 5b displays the Tauc plots ((αhv)2 plot versus photon energy ()) for calculation of the ZnO:Ag thin films band gaps.

Thus, the optical band gap energy is found to be 3.22 eV for pure ZnO thin film which is smaller than that of the bulk ZnO (3.3 eV). The same value was previously published by Tarwal et al. in reference [35] when they synthesized Ag-doped ZnO thin films by spray pyrolysis technique. When the Ag content increases from 1% Ag to 2% Ag, the optical band gap is first found to increase from 3.239 to 3.245 eV. Subsequent increase of Ag doping to 5% Ag reduces the band gap value to 3.231 eV. Indeed, the same behaviour was assessed in the sol–gel-deposited Ag-doped ZnO thin films prepared by Chelouche et al. in reference [65]. Therein, they found that the optical band gap of 2% Ag-doped ZnO thin films was increased while the optical band gaps of 1 and 3% Ag-doped ZnO thin films were decreased as compared to those of ZnOpure thin film. The blueshift of the band gap may be, on the one hand, due to the Burstein–Moss effect occurring since Ag incorporation in the ZnO lattice. Ag atoms probably worked as ionized donors yielding one extra electron. This electron occupied the bottom of the conduction band. Thus, the number of electrons increased as the doping concentration was increased, which widened the energy gap. A similar trend has been reported in sputtered Ag-doped ZnO thin films prepared by Xue et al. [66]. Herein they found that the optical band gaps of the films all increased by Ag doping. On the other hand, the decrease of E g for higher Ag doping may be attributed to segregation of Ag atoms to grain boundaries. These atoms introduce electron-localized states at the ZnO band gap closer to the lower edge of conduction band forming new lowest unoccupied molecular orbital, which leads to the band gap reduction. The decrease of the band gap with increasing Ag doping in ZnO has been also revealed in several previous reported results. For instance, Karyaoui et al. have published in reference [42] that the energy band gaps of Ag-doped ZnO-sprayed thin films have decreased with Ag doping from 3.29 eV for undoped ZnO to 3.25 eV for ZnO doped with 3% Ag. In addition, Sahu et al. [63] have found a narrowed band gap with increasing Ag content in magnetron-sputtered ZnO thin films. In contrast, Xu et al. [67] have reported that the incorporation of Ag had almost no influence on the optical band gaps of ZnO thin films and concluded that the optical band gap of ZnO thin films is strongly dependent on the lattice sites of Ag in ZnO which in turn is dependent on films deposition methods and annealing treatments.

thumbnail Fig. 5

(a) Optical transmission spectra of pure and Ag-doped ZnO thin films and (b) Tauc plots for calculation of the ZnO:Ag thin films band gaps.

3.5 Photoluminescence studies

The PL spectra of silver-doped ZnO thin films are recorded at room temperature and displayed in Figure 6.

It is worth noting from Figure 6 that a sharp violet peak located at about 378.5 nm was observed for pure ZnO thin film. However, it down-shifted to about 378 nm after Ag doping. In the literature, there is a consensus that the UV emission of ZnO belongs to the near band edge emission (NBE) since its position is located close to the band gap energy (∼3.3 eV) of ZnO at room temperature [68] and it was attributed to the radiative recombination of free excitons [45].

It is also remarkable that the intensity of this peak decreased with the increase in Ag doping content from 1 to 2%, and then increased with further increase in Ag amount up to 5%. This behaviour is totally consistent to the PL results found by Tripathi et al. [53], where the intensities of the UV peak for the sol–gel prepared with 0, 5, 10 and 15% Ag-doped ZnO thin films increase as Ag doping concentration increases respectively except for the 2% Ag-doped thin film.

thumbnail Fig. 6

Photoluminescence spectra of pure and Ag-doped ZnO thin films.

3.6 Correlation between UV-vis transmittance and photoluminescence measurements of Ag-doped ZnO

The gap energies of silver-doped ZnO thin films could be derived from optical measurements. All the estimated values are listed in Table 3. Figure 7 depicts the evolution trend for both the energy band gap (Eg ) and the position of the UV emission peak as a function of Ag contents. Therefore, optical band gap energies estimated from UV-vis transmittance data Eg (opt.) were found in the range of 3.36–3.38 eV. Meanwhile, band gap values obtained from PL spectra were slightly changed and found to be approximately in the range of 3.283–3.288 eV. Figure 8 showcases the optical transmission and PL intensity as a function of Ag contents of silver-doped ZnO thin films. A clearly band gap redshift between Eg (opt.) and Eg (PL) was noted for the whole studied films. This redshift mainly arises from either intrinsic or extrinsic defects in the material which can easily form recombination centres. Thus, these defects introduce levels inside the band gap of semiconductor material.

Table 3

Energy band gap (Eg) and position of UV emission peak at different Ag content.

thumbnail Fig. 7

Energy band gap (Eg) and position of UV emission peak as a function of Ag content.

thumbnail Fig. 8

Optical transmission and PL intensity as a function of Ag contents for (a) undoped ZnO, (b) ZnO:Ag (1%), (c)ZnO:Ag (2%) and (d) ZnO:Ag (5%) thin films.

3.7 Photoelectrochemical performance

Figure 9 depicts current density–potential (JV) curves of undoped and Ag-doped ZnO films in 0.5 M aqueous Na2SO4 solution in the dark and under illumination.

The most crucial figures of merit distinguishing the performance of a PEC solar cell are its short-circuit current (I sc), open-circuit voltage (V oc) and fill factor (FF). I sc is the maximum current flowing through the PEC solar cell when the voltage is equal to zero (when the PEC solar cell is short-circuited). V oc is the maximum voltage when the current is zero (when the PEC solar cell is in the open-circuit condition). These two output parameters can be extracted from plots of current–voltage curves illustrated in Figure 9. These parameters are listed in Table 4.

The FF is the ratio of the maximum power (P max) from a PEC solar cell to the product of I sc and V oc and can be calculated using equation (6) [69]:(6)where I max and V max are the maximum current and voltage, respectively, I sc is the short-circuit current and V oc is the open-circuit potential that can be extracted from the (JV) curves.

Both V oc and I sc values of PEC cell are plotted against the Ag doping percentage in Figure 10.

It can be seen from Figures 9 and 10 that the obtained values of the open-circuit voltage (V oc) and short-circuit current (I sc) for undoped ZnO thin film under illumination are 326 mV and 27.92 μA, respectively. With the rise in Ag doping level, I sc values increase gradually from 33.7 μA for 1% Ag up to 49.3 μA for 2% Ag. Then, this current value decreases to 22.03 μA for 5% Ag doping content. However, there is a drop in terms of V oc attaining a minimum value of 132 mV for 5% Ag doping concentration.

Table 4

PEC parameters of the cell with “glass/ITO/Ag-ZnO/SCE” configuration.

thumbnail Fig. 9

Photocurrent–voltage curves in the dark and under visible light (200 mW/cm2) for (a) undoped ZnO, (b) ZnO:Ag (1%), (c) ZnO:Ag (2%) and (d) ZnO:Ag (5%) thin films in 0.5 Na2SO4 electrolyte.

thumbnail Fig. 10

Variation of I sc and V oc for the PEC cell formed with Ag:ZnO thin films versus Ag doping content.

3.8 Correlation between photoelectrochemical and photoluminescence measurements of Ag-doped ZnO/ITO photoanode

To enrich the present investigation, a correlation between PEC and PL measurements of Ag-doped ZnO/ITO photoanode has been developed. From Figure 11 depicting the variation of I PL and I sc as a function of Ag doping contents, it has been emphasized that the change trend of I PL and I sc are almost opposite.

It is noticeable that the UV emission intensity decreases with increasing Ag content up to 2% and then increased as the Ag concentration further increased to 5%. The decrease of the UV emission peaks when Ag doping is increased may be assigned to the increase of the lattice defects.

Meanwhile, with the rise of Ag doping content, I sc values increase gradually from 33.7 μA for 1% Ag up to 49.3 μA for 2% Ag. With further increase in Ag doping content, current value decreases with a value of I sc about 22.035 μA for 5% Ag-doped ZnO film. Thus, the increment of I sc values with increasing the Ag amount can be mainly explained with a possible photogenerated charge separation improvement due to the electric field induced in the depletion region. Thus, the radiative recombination probability was decreased.

thumbnail Fig. 11

PL intensity (I PL) and short-circuit current I sc as a function of Ag contents.

3.9 Electrochemical impedance spectroscopy analysis

Electrochemical impedance spectroscopy (EIS) is a well-known technique that is widely exploited to study the impedance spectra (Nyquist plots) for the ZnO:Ag electrodes immersed in 0.5 M Na2SO4 electrolyte at an AC frequency varying from 105 to 0.01 Hz are reported in Figure 11.

The Nyquist diagrams of the undoped and Ag-doped ZnO electrodes ((Z″) versus (Z′)) are illustrated in Figure 12. By analysing the obtained EIS spectra, the impedance arc radii of the undoped ZnO electrode (imaginary impedance (Z″) versus real impedance (Z′) under illumination are much smaller than that plotted in the dark.

As it can be seen, the Nyquist plots of the undoped and Ag-doped ZnO electrodes under illumination feature a semicircle curves with their diameters changing upon varying the amount of Ag. This suggests a nearly capacitive behaviour of these electrodes. In terms of the Bode diagram (Fig. 13), this behaviour can be seen through modulus |Z| values versus log frequency plot (Fig. 13a) as an oblique lines at low frequencies followed by a horizontal line over the whole range of high frequencies. Clearly, the modulus |Z| values at the low-frequency limit (10 mHz) decreased significantly after doping ZnO with Ag compared to those undoped samples. Similar behaviour can be seen in the phase angle versus log frequency plot as a function of the amount of Ag (Fig. 13b). As illustrated in this figure, the phase angle magnitude remains constant in the vicinity of 60°–73° with a small shift towards higher frequencies.

The EIS results can be fitted by the equivalent circuit presented in Figure 14 and the fitted parameters are listed in Table 5.

In this circuit, R s is the electrolyte resistance which can be calculated by the intercept of semicircles on the real axis (Z′) at high frequency. However, at low frequency, the intersection at real axis (Z′) corresponds to the sum of electrolyte resistance (R s) and the charge transfer resistance (R ct) originated from electrons diffusion. C d corresponds to the double-layer capacitor. Also, Warburg impedance (Z W) is the electrolyte diffusion impedance and associated with the redox species diffusion in the electrolyte.

thumbnail Fig. 12

Nyquist plots for the Ag-doped ZnO electrodes immersed in 0.5 Na2SO4 electrolyte.

thumbnail Fig. 13

(a) Impedance modulus |Z| and (b) phase angle variation versus log frequency as a function of Ag content.

thumbnail Fig. 14

(a) Equivalent circuit for undoped ZnO thin films under illumination. (b) The equivalent circuit of Ag-doped ZnO thin films under illumination.

Table 5

Equivalent circuit parameters for undoped and Ag-doped ZnO films under illumination in Na2SO4 electrolyte.

4 Conclusion

Silver-doped ZnO thin films have been successfully prepared using sol–gel process onto ITO-coated glass substrates. The effect of Ag doping content (1, 2 and 5%) on structural, morphological, optical and PEC properties was studied. It was found that Ag doping leads to enhanced grain sizes, thickness and band gaps of elaborated SZO thin films. The PEC performance of the SZO films are improved as compared to the undoped ZnO and the film containing 2% Ag yielded the highest photocurrent at about 49.34 μA. A correlation study between PEC and PL measurements of Ag-doped ZnO/ITO photoanode has been developed and largely discussed. The 2% Ag-doped ZnO photoanode has the minimum PL intensity and the best response in PEC. Thus, silver at this 2% Ag doping content has remained on surface leading to the separation of electron–hole due to the electric field at the Ag–ZnO interface. The 5% Ag-doped ZnO has the maximum PL intensity and the minimum PEC response at around 22 μA was obtained. Hence, among all the obtained results, 2% Ag-doped ZnO photoanode may be used as efficient photoelectrode for thin film-based solar cell devices.

Acknowledgments

This work was funded by the Ministry of Higher Education and Scientific Research of Tunisia.

Author contribution statement

The wide majority of the work has been carried out by the Ph.D. student DRIDI Donia (Université de Carthage, Faculté des sciences de Bizerte Tunis, Tunisia) under the direction of Prof. Chtourou Radhouane, Director of Laboratory of Nanomaterials and Systems for Renewable Energies (LaNSER), Research and Technology Center of Energy (Technopole Borj‑Cedria, Hammam‑Lif, Tunis), supervised by Yousra Litaiem, Associate Professor in Laboratory of Nanomaterials and Systems for Renewable Energies (LaNSER), who played an essential role of counselling during both the research and the redaction phase and Mokhtar Karyaoui, Associate Professor in Laboratory of Nanomaterials and Systems for Renewable Energies (LaNSER), who played an essential role of counselling during research phase.

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Cite this article as: Donia Dridi, Yousra Litaiem, Mokhtar Karyaoui, Radhouane Chtourou, Correlation between photoelectrochemical and photoluminescence measurements of Ag-doped ZnO/ITO photoanode, Eur. Phys. J. Appl. Phys. 85, 20401 (2019)

All Tables

Table 1

The microstructural parameters evaluated from XRD measurements such as position peak, lattice parameters (a, c), grain size (D), microstrain ϵ and density of dislocation δ for Ag-doped ZnO thin films.

Table 2

Thicknesses of undoped and Ag-doped ZnO thin films.

Table 3

Energy band gap (Eg) and position of UV emission peak at different Ag content.

Table 4

PEC parameters of the cell with “glass/ITO/Ag-ZnO/SCE” configuration.

Table 5

Equivalent circuit parameters for undoped and Ag-doped ZnO films under illumination in Na2SO4 electrolyte.

All Figures

thumbnail Fig. 1

Schematic diagram showing photoelectrochemical current measurement.

In the text
thumbnail Fig. 2

XRD spectra of Ag-doped ZnO thin films deposited on ITO substrates at different Ag contents (Ag = 0, 1, 2 and 5%).

In the text
thumbnail Fig. 3

Crystallite size and residual stress of silver-doped ZnO thin films.

In the text
thumbnail Fig. 4

SEM micrographs of the sol–gel-deposited Ag-doped ZnO thin films at various Ag contents. (a) ZnOpure (b) ZnO:Ag 2% and (c) ZnO:Ag 5%.

In the text
thumbnail Fig. 5

(a) Optical transmission spectra of pure and Ag-doped ZnO thin films and (b) Tauc plots for calculation of the ZnO:Ag thin films band gaps.

In the text
thumbnail Fig. 6

Photoluminescence spectra of pure and Ag-doped ZnO thin films.

In the text
thumbnail Fig. 7

Energy band gap (Eg) and position of UV emission peak as a function of Ag content.

In the text
thumbnail Fig. 8

Optical transmission and PL intensity as a function of Ag contents for (a) undoped ZnO, (b) ZnO:Ag (1%), (c)ZnO:Ag (2%) and (d) ZnO:Ag (5%) thin films.

In the text
thumbnail Fig. 9

Photocurrent–voltage curves in the dark and under visible light (200 mW/cm2) for (a) undoped ZnO, (b) ZnO:Ag (1%), (c) ZnO:Ag (2%) and (d) ZnO:Ag (5%) thin films in 0.5 Na2SO4 electrolyte.

In the text
thumbnail Fig. 10

Variation of I sc and V oc for the PEC cell formed with Ag:ZnO thin films versus Ag doping content.

In the text
thumbnail Fig. 11

PL intensity (I PL) and short-circuit current I sc as a function of Ag contents.

In the text
thumbnail Fig. 12

Nyquist plots for the Ag-doped ZnO electrodes immersed in 0.5 Na2SO4 electrolyte.

In the text
thumbnail Fig. 13

(a) Impedance modulus |Z| and (b) phase angle variation versus log frequency as a function of Ag content.

In the text
thumbnail Fig. 14

(a) Equivalent circuit for undoped ZnO thin films under illumination. (b) The equivalent circuit of Ag-doped ZnO thin films under illumination.

In the text

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